This invention relates to magneto-optical recording media for reproducing information signals by virtue of magneto-optical effects. More particularly, the invention relates to a magneto-optical recording medium in which it is possible to set the curie temperature of a recording layer at a high level.
Recently, for example, in the field of recording media used for computers or the like, a demand is apparent for expansion of the recording capacity of a recording medium which is used as an external memory, with increase of information to be processed. Further, in the field of conventional magneto-optical recording and reproducing apparatus on ISO format, a red laser diode that emits a laser whose wavelength is 680 nm has been used, and systems which have a recording capacity four times as large as that of the conventional machine are available in the market. Furthermore, a demand for systems which allow a recording capacity eight times as large as that of the conventional machine has become acute.
The magneto-optical disk which is used for such a recording and reproducing apparatus is constructed, as shown in FIG. 1, such that a substrate 100 which is made of PC (polycarbonate) or glass transmissible to a laser beam has a lower dielectric layer 101, a recording film 102, an upper dielectric layer 103, a reflective film 104 and a protective film 105 formed one over the other successively in this order. These the lower dielectric layer 101 and the upper dielectric layer 103 are composed, for example, of silicone nitride (SiN), the recording film 102 is composed, for example, of a TbFeCo amorphous compound, the reflective film 104 is composed, for example, of Al, and the protective film is composed, for example, of a UV curing resin.
With this magneto-optical disk, when information signals are recorded on the recording film 102, a laser beam is applied to the recording film 102. This leads to the increase of temperature of the recording film 102, and when the temperature of the recording film 102 becomes higher than its curie temperature, the magnetism of the recording film 102 disappears. Later, during a process whereby the recording film is cooled, a specified external magnetic field is applied to that recording film 102, and, through this process, magnetising of the recording film 102 takes place in a desired direction, and thus a record mark having a specified magnetising is formed.
For the magneto-optical disk working on above principle to acquire a large capacity to record signals, it is necessary reduction in the size of the record mark. However, with the magneto-optical disk, to reduce the size of the record mark will lead to the deterioration of signal quantity. On the other hand, with a magneto-optical disk incorporating a recording film 102 made of rare earth metals/transition metals, it is possible to improve signal reproduction by raising the curie temperature of the recording film 102. With this magneto-optical disk, it is possible to compensate for the deteriorated signal capacity due to the contracted record mark by incorporating a recording film 102 whose curie temperature has been set high. With such magneto-optical disk, it is possible to set the curie temperature of the recording film 102 at a desired level by appropriately altering the composition of elements constituting the recording film 102.
Here reference is made to FIG. 2 which illustrates a characteristic graph relating the curie temperature of a recording layer 102 and Kerr's angle of rotation of the disk with a four-layered structure. This Kerr's angle of rotation gives a value proportional to signal quantity. As is obvious from FIG. 2, the higher the curie temperature of the recording film 102, the larger the Kerr's angle of rotation. Namely, it is possible to enlarge signal quantity by setting the curie temperature of the recording film at a high level.
With the above-described magneto-optical disk, however, if the curie temperature of the recording film 102 is set high to enlarge signal quantity, it will be necessary to raise the temperature of the recording film 102 by the same degree during recording. With the magneto-optical disk, when information signals are recorded, the recording film 102 temperature is raised until it is not less than the curie temperature, and thus a large amount of calories will be transmitted to the substrate 100. As a result, with the magneto-optical disk of such constitution, deterioration of the substrate due to heat will ensue. With the magneto-optical disk, if deterioration of the substrate takes place, the so-called repetitive recording and reproducing performance will be deteriorated, too. To be more specific, with a conventional magneto-optical disk which incorporates a recording film 102 made of rare earth metals/transition metals, it is customary to set the curie temperature at around 180.degree. C. at highest.
Accordingly, with the conventional magneto-optical disk as described above, it is difficult to set the curie temperature of the recording film 102 at a high level, thereby to enlarge signal quantity, and thus to record on the basis of minute record marks enabling a high density recording.